Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Further, wind turbine power systems may include a variety of generator types, including but not limited to a doubly-fed induction generator (DFIG).
DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the power converter can provide nearly instantaneous regulation of its output currents with respect to the grid frequency. Under steady operating conditions, the rotor-side converter controls the magnitude and phase of currents in the rotor circuit to achieve desired values of electromagnetic torque. Reactive power flow into the line-connected stator terminals of the generator can also be controlled.
Such DFIG wind turbines may or may not be equipped with a dynamic brake that includes parallel insulated-gate bipolar transistors (IGBTs) which feed power into a resistor. Minimum components for the dynamic brake typically include a switch (typically a semiconductor such as an IGBT) and a resistor and may also include one or more diode(s) in parallel with either the switch, the resistor, or both, as well as other components. Without dynamic braking, typical operation of a DFIG wind turbine is configured to regulate the positive sequence voltage with a closed-loop current regulation scheme which minimizes negative sequence current. As the length of the transmission line feeder to the DFIG wind turbine is increased, however, response to grid transients and grid disturbances causes oscillations of power into and out of the power converter, which can create disturbances on the DC bus voltage therein. As longer transmission line length is typically desired (and possibly coupled with larger grid voltage transients), the voltage overshoots on the DC bus voltage in the power converter may reach a level that damages the converter components. Thus, the dynamic brake may be used to control the peak voltage on the DC bus.
For conventional dynamic brakes, controls for the switch may be operated based solely on the level of the DC bus voltage in the power converter. As converter power levels continue to increase, additional IGBTs must be placed in parallel to conduct the current. Therefore, it is important to balance the loss in the parallel IGBTs because the loss directly impacts the junction temperature, and the IGBT with the highest junction temperature is the limit in the total energy that can be fed into the resistor.
Thus, the present disclosure is directed to an improved dynamic brake circuit assembly for a wind turbine that addresses the aforementioned issues.